Ceramic molds for metal casting can be created directly from a computer model using layer techniques, i.e., three dimensional printing processes which can be defined as processes that construct objects in layers using a computer model of the objects. Exemplary processes of this type are described, for example, in U.S. patent application Ser. No. 07/447,677, filed on Dec. 8, 1989 , now U.S. Pat. 5,204,055 which application is incorporated by reference herein. As described therein, a ceramic mold for metal casting can be created directly from a computer model using such process wherein the mold is created on a layer by layer basis. As shown specifically in FIGS. 1(A)-1(F) herein, the mold may be created by spreading powder 1 using roller 3 within a confined region as defined by a piston 5 and cylinder 7 arrangement. A further material 9, e.g. a binder material, is then deposited at specific regions 11 of a layer as determined by a computer model of the mold. The further material acts to bind the powder within the layer and between layers. This process is repeated layer after layer until all layers needed to define the mold have been printed. The process results in a bed of powder 13 which contains within it a ceramic mold 15. When the further material which is used to bind the powder contains a ceramic, the entire bed can be fired at an elevated temperature. Next, the powder on the exterior surfaces of the mold is removed to provide a mold which is still filled with loose powder on the interior surface 17 thereof. The loose interior powder must then be removed to yield a hollow mold 19. As the passageways within the mold can be long and complex, the geometry often precludes the use of a tool such as a brush to aid in interior powder removal. One technique for removing the powder which is disclosed in the above application is to wash or flush the powder from the interior surface. In some cases, and particularly when the further material used to bind the powder contains a polymer, the mold can be removed from the powder bed before firing.
It will be understood that other methods might also be used to create ceramic molds directly from a computer model. For example, selective laser sintering might also be used to create such a mold from ceramic powder. Thus, the current invention can be applied to molds made directly from a computer model, regardless of the process used.
A major problem with such technique is that the powder is not always thoroughly removed when a mere flushing operation is used. Accordingly, it is desirable that other techniques be devised to provide more complete and efficient removal of the powder from the interior of the mold.
Moreover, another major problem that arises is that, since the mold is fabricated from a plurality of layers, a non-smooth surface, e.g. a "stair-stepped" surface on the interior of the mold is usually produced during the layering process. While the magnitude of this surface effect can be reduced, by reducing the layer thickness, an undesirable increase in the fabrication time results. Acceptable surface finish is, therefore, not always achievable when using a practical fabrication time cycle. It is desirable, therefore, to devise techniques to achieve the desired surface finish utilizing further operations after the mold has been fabricated and the powder is removed from the internal passages.